(1) Field of the Invention
The present invention relates to methods for forming porous layers on the outer surfaces of electrode hoops of nonaqueous electrolyte secondary batteries or other batteries.
(2) Description of Related Art
As portable electronic devices and cordless electronic devices have come to be more widely used, expectations are growing for lightweight nonaqueous electrolyte secondary batteries with high energy density. However, an active material of such a nonaqueous electrolyte secondary battery is made of highly reactive lithium, and therefore a short circuit between a positive electrode and a negative electrode under abnormal conditions generates heat. Due to the generated heat, a resinoid microporous membrane separator for isolating the positive electrode from the negative electrode melts around the area in which the electrodes are short-circuited, thereby increasing the area in which the electrodes are short-circuited and thus generating abnormal superheat. In order to prevent this, a technique is being used in which, in case of the occurrence of a short circuit between electrodes, superheat due to an increase in the area where the electrodes are short-circuited is suppressed by combining a porous refractory layer having a refractory resin or an inorganic oxide as the main ingredient with the resinoid microporous membrane separator.
Such a porous refractory layer is formed on the outer surface of a positive electrode or a negative electrode (hereinafter, referred to as an “electrode”) without loss of the design capacity of a battery to have a thickness of 2 through 10 μm. A method in which a coating fluid serving as a precursor of a porous refractory layer is transferred to a gravure roll provided with a plurality of grooves and the transferred coating fluid is applied to the outer surface of a target electrode hoop (hereinafter, referred to as “gravure method”) is preferably employed as a method for forming a layer of a thickness as described above with high accuracy. The electrode hoop formed at its outer surface with a porous refractory layer is then cut into pieces each having a necessary size, and the resultant pieces are utilized as electrodes for batteries.
In order to realize the gravure method, the following two techniques are effective. One of the techniques is that the direction in which an electrode hoop travels is allowed to become opposite to the direction of rotation of a gravure roll. In this way, a thin coating membrane can be formed on the outer surface of an object to be coated with a coating fluid with high accuracy. The other one of the techniques is that a plurality of grooves are formed in the circumferential surface of a gravure roll to extend downward from one outer edge of the circumferential surface to the other outer edge and generally in parallel. This method allows a coating fluid to be uniformly spread out over the outer surface of an object to be coated with the coating fluid. Therefore, a thin coating membrane can be formed with higher accuracy.
FIG. 6 is a schematic view partially illustrating the state in which a porous refractory layer is formed using a known gravure roll. In FIG. 6, the circumferential surface of the gravure roll 1 and respective cross sections of a mixture layer 3 of an electrode hoop and a porous refractory layer 4 are illustrated. When a plurality of grooves 7a are formed to extend downward from one outer edge of the circumferential surface of the gravure roll 1 to the other outer edge (from top left to bottom right in FIG. 6) and generally in parallel with one another, a coating fluid totally flows from left to right. The coating fluid used in the gravure method is a Newtonian fluid having a low viscosity. Therefore, the resultant porous refractory layer 4 is formed necessarily with a thin part represented as A and a thick part represented as B.
When a thin roll or the like is used, as an object to be coated with a coating fluid, in the gravure method, a porous refractory layer is formed with a continuously extending thick part. This deforms the roll when the object coated with the coating fluid is rolled up again to take the form of rolls.
A coating method in which a porous refractory layer is prevented from being formed with a thick part by changing the angle of inclination of each of grooves only in a region of a gravure roll having a predetermined width from one outer edge of the circumferential surface of the gravure roll has been suggested in Japanese Unexamined Patent Publication No. 11-005052.